1. Field of the Invention
The present invention relates to a projection exposure apparatus for transferring a mask pattern such as a circuit pattern on a photosensitive substrate, more particularly, it relates to a projection exposure apparatus provided with a focus detection device for focusing on the photosensitive substrate.
2. Related Background Arts
In the conventional projection exposure apparatus, when a pattern of a reticle (a photomask or the like) is projected onto a photosensitive substrate (a wafer, a glass plate or the like on which a photoresist layer is coated) through a projection optical system with a high resolution to expose the substrate, an exposure surface on the photosensitive substrate must be accurately coincided with the image plane of the pattern on the reticle, that is focusing of the pattern of the reticle is necessary. Recently, the focal depth of the projection optical system tends to become narrow. However, a dept of only about .+-.0.7 .mu.m can be obtained under the present state though i line having the wavelength of 365 nm is used as an illumination light for exposure. Moreover, a projection visual field of the projection optical system tends to expand year by year, so that it is desirable that a largest focal depth is obtained in all of a board exposure visual field (for example, a square having side of 22 mm).
To achieve satisfactory focusing across the broad exposure visual field, a better flatness of a partial area (shot area) on the photosensitive substrate within the exposure visual field and a better flatness of the image plane (that is, the curvature and the inclination of the image plane are small) must be obtained. The curvature and the inclination of the image plane are mainly dependent on the optical performance of the projection optical system, in addition, they are sometimes dependent on the flatness of the reticle and/or the parallelism between the reticle and the substrate. On the other hand, the flatness of the partial area on the photosensitive substrate, that is the flatness of every projection exposure area (shot area) differs from substrate to substrate. However, the surface in the shot area on the photosensitive substrate can be set in parallel with the image plane by inclining a holder for holding the photosensitive substrate by a small angle.
The methods for performing focusing under consideration of the surface inclination in a one shot area on the photosensitive substrate are disclosed in Japanese Patent Application Laid-Open No. Sho 58-113706 (U.S. Pat. No. 4,558,979) and Japanese Patent Application Laid-Open No. Sho 55-1348 (U.S. Pat. No. 4,383,757). Particularly, U.S. Pat. No. 4,383,757 discloses the technique, wherein spots of the light beam are projected at four points on a photosensitive substrate through a projection optical system and then a spot image formed by the reflected light is photo-detected to carry out focusing of the photosensitive substrate and correction of the inclination thereof (leveling).
However, since recent semiconductor devices are manufactured by superposing many complex structure patterns on a substrate, the flatness of the exposure surface on the photosensitive substrate becomes worse. Therefore, a technique has been developed wherein an irregular condition in the shot area on the photosensitive substrate is measured and then an average surface in the shot area is focused onto the image plane by the projection optical system based on the measured result. For example, Japanese Patent Application Laid-Open No. Hei 2-198130 (U.S. Pat. No. 5,124,562) discloses a surface position detecting method, wherein photosensitive substrate is fixed in the direction along the optical axis of the projective optical system but moved in the direction perpendicular to the optical axis, the positions (focus position) in the direction along the optical axis of the projective optical system are measured at a plurality of measurement points in the shot area on the photosensitive substrate, and then the average of the measured results is obtained, whereby the offset value of the focus position, which is based on the differences in structure and/or position of the patterns in the shot area, is obtained. In this method, the average focus position is measured in consideration of the irregularities in the shot area by adding the offset value to the measured result of the focus position at each shot area, for example, at the center measurement point.
As described above, in the conventional projection exposure apparatus, the offset value of the focus position is obtained by averaging the focus positions measured at a plurality of specific measurement points. However, in practice, the irregular conditions of the exposure surface in every shot area on the photosensitive substrate vary in accordance with the process construction (such as arrangements and differences in level of patterns), so that, the average surface shape at every shot area can not be obtained accurately only by averaging the focus position at a plurality of specific measurement points. Therefore, if the arrangement, the differences in level and the like of the pattern in every shot area on the photosensitive substrate change, there is a disadvantage that the average surface in every shot area can sometimes not be placed within a range of the focal depth with respect to the image plane of the projection optical system.
Moreover, it is difficult to conform the area to the image plane by the conventional method even when the average surface in every shot does not conform to the image plane, and, an area within the shot area, in which a pattern with the narrowest line width is exposed, is mainly conformed to the image plane.